The present invention relates to a light diffusion sheet for use in a backlight unit incorporated in a liquid crystal display (LCD) and to a backlight unit using a light diffusion sheet.
In the field of LCDs, backlight systems have been widely utilized for illuminating a liquid crystal layer from its back for light emission. A known LCD has a backlight unit placed on the underside of a liquid crystal layer. Such a backlight unit 20 is generally comprised of, as shown in FIG. 8, a bar-like lamp 21 (light source); a light guide plate 22, in the form of a square plate, placed with one end extending along the lamp 21; a light diffusion sheet 23 placed on the surface of the plate 22; and a prism sheet 24 placed on the surface of the sheet 23.
In the backlight unit 20, light beams (ray) emitted from the lamp 21 and incident on the plate 22 are reflected off the side faces of the plate 22 and reflective dots or a reflective sheet (not shown) on the rear face of the plate 22 and then go out of the surface of the plate 22. The light beams (ray) transmitted from the plate 22 are incident on the sheet 23, diffused, and then, go out of the surface of the sheet 23. Thereafter, the light beams from the sheet 23 are transmitted to the prism sheet 24 and then allowed by a prism section 24a formed on the surface of the sheet 24 to go out as light beams that form a distribution with a peak at an upward direction substantially normal to the sheet 24. The light beams from the lamp 21 are diffused by the sheet 23 and, then, refracted by the sheet 24 so as to form a distribution peak at an upward direction substantially normal to the sheet 24 to illuminate the entire surface of the liquid crystal layer (not shown) disposed in an upper position.
With reference to FIG. 9, there is provided another type of backlight unit 20 which is designed, in view of the light collecting properties of the prism sheet 24, such that another light diffusion sheet 23 or prism sheet is disposed on the surface of the sheet 24.
The known light diffusion sheet 23 commonly employed in the backlight unit 20 comprises, as shown in FIG. 10, a light diffusion layer 26 containing beads 29 dispersed within a binder 28; a transparent base material sheet 25; and a sticking-inhibiting layer 27 containing beads 31 dispersed within a binder 30, light diffusion layer 26, base material sheet 25 and sticking-inhibiting layer 27 are laminated in this order.
The binder 30 used for the sticking-inhibiting layer 27 is poor in wear resistance and liable to scratch defects. In addition, since the beads 31 are dispersed in the binder 30, there is a risk that the rear face of the light diffusion sheet 23 is easily damaged or deformed, resulting in depreciation of its commercial value, if a plurality of light diffusion sheets 23 are stacked together on top of one another during manufacture, delivery, storage, and the like, of the light diffusion sheets or if the light diffusion sheet 23 is laid over the irregular surface of the prism sheet, when incorporated into the backlight unit 20.
The present inventors have made the present invention in consideration of the problems realized in the prior art by intensively investigating the materials to be employed in the light diffusion layer and sticking-inhibiting layer of the light diffusion sheet. Specifically, an object of the invention is to provide a light diffusion sheet comprising a transparent base material sheet; a light diffusion layer laminated on a surface of the base material sheet and containing beads dispersed within a binder; and a sticking-inhibiting layer laminated on a rear face of the base material sheet, wherein the binder of the light diffusion layer contains a thermosetting resin and the sticking-inhibiting layer is a resin layer containing an ionizing radiation curable resin In the light diffusion sheet of the invention, which employs an ionizing radiation curable resin for the binder of the sticking-inhibiting layer, the cross-linking density of the binder can be increased, whereby the wear resistance of the sticking-inhibiting layer as well as the wear resistance of the rear face of the light diffusion sheet can be increased. Further, possible occurrence of scratches in the sticking-inhibiting layer, which is attributable to the contact between the sticking-inhibiting layer and the beads of the light diffusion layer, can be effectively avoided. According to the invention, even if a plurality of light diffusion sheets, which have the light diffusion layer containing beads dispersed within the binder or projecting from the surface of the binder, are stacked together on top of one another during delivery or other occasions, scratch defects can be effectively prevented in the sticking-inhibiting layer located on the rear face of the light diffusion sheet, with the result that degradation of optical properties such as decreases in light beam transmissivity caused by scratches in the sticking-inhibiting layer can be avoided and therefore desired optical properties can be ensured.
In addition, beads may be dispersed within the sticking-inhibiting layer in order to enhance light diffusibility.
According to a preferred embodiment of the invention, the light diffusion sheet may further contain a wax whose particles are dispersed within the light diffusion layer and/or the sticking-inhibiting layer. Specifically, in the light diffusion sheet of this embodiment, light beams (ray) passing through the interfaces of the particles of the wax dispersed within the binder of the light diffusion layer are refracted in various directions, so that not only improved light diffusibility can be achieved but also scratch defects caused by the contact between the sheet surfaces when the light diffusion sheets are used in a stacked condition can be prevented. The particles of the wax dispersed within the binder are melted by a pressure or friction heat that acts upon the surface of the light diffusion sheet, so that they are present on the surface of the light diffusion sheet as a lubricant oil or crystallized extending in various directions according to the force acting on the surface of the light diffusion sheet. As a result, slippage, blocking resistance, and wear resistance are imparted to the light diffusion sheet, increasing the scratch prevention capability of the sheet.
According to another preferred embodiment of the invention, the light diffusion sheet may further include a micro inorganic filler whose particles are dispersed within the light diffusion layer and/or the sticking-inhibiting layer. More specifically, the light diffusion sheet of this embodiment contains not only beads but also a micro inorganic filler within the light diffusion layer and/or the sticking-inhibiting layer, so that the apparent crystallinity and therefore heat resistance of the light diffusion sheet can be improved. Consequently, the deflection of the light diffusion sheet when the lamp is lit can be restricted.
According to the invention, there is provided a backlight unit for use in a liquid crystal display, the unit comprising a lamp; a light guide plate disposed beside the lamp for guiding light beams to the front side of the light guide plate, the light beams having been emitted from the lamp; and a light diffusion sheet disposed on the front side of the light guide plate. The use of the light diffusion sheet of the invention enables it to assure improved wear resistance, heat resistance and the like, to prevent scratch defects and to remarkably facilitate handling of not only the light diffusion sheet but also the backlight unit during manufacture, delivery, storage and other occasions.